schematic for linear pelt psu
 

preferably adjustable voltage, around 30 amp output. If that is even possible.

anyone know where I can get my hands on such a drawing?

anything close to it even?

I think I have seen similar schematics on this site but my searches yielded nothing.

There is an article at overclockers that describes building a linear supply for a TEC. It's not adjustable though.

Doing it right though will be more expensive than just buying a switcher. The efficiency will suck too.

To vary the output of such a linear supply, you would want to connect a variac to its input. Regulating the output voltage on the output side of the supply will dissipate way too much power. You need to regulate the AC input. That is what the variac is for.

BTW, I happen to have a variac available that could do the job.

Oooh, I like the idea of using a variac like that! And with TEC, you could probably get away with just rectifying and filtering, no regulation.

By the by, Since, how big is that variac? I've an old 10 A one, but it's too bulky for casual use. One last bit of hijacking - don't suppose you have any info on brush replacing/rebuilding? Mine's getting mighty worn.

TEC manufacturers recommend keeping the ripple under 10% for fullwave rectified 60Hz, so it's a lot of filtering. The caps are one of the major expenses for doing this.

The variac I have is 5A. Probably enough for a 12V, 30A supply, as long as the efficiency of the linear is reasonable. Definitely enough for a 226 Watt TEC supply.

Edit: Don't know anything about replacing the brush.

BTW here's the link to the Overclockers article.

And, here's a link to a thread discussing it. (I had some issues with the article.)

10%, eh? Is that spec'd peak to peak, or RMS. Either way, that'd be a lot of caps.

Bummer.

http://www.onsemi.com/site/products/...MR2510,00.html

Check those bad boys out. You could probably build a bridge rectifier without too much trouble with those, and a relatively large capacitor.

EDIT: Say, a 0.5F 15V capacitor, or something like that, which is relatively cheap at surplus stores and stuff (there's a store here in town that has lots of high capacitance low voltage caps).

I don't think the place I saw that recommendation specified P-P or RMS, but yeah, a lot of capacitance anyway.

Scrounging at surplus places might work, but I'd be concerned about the shelf life of old electrolytic caps. (I wouldn't want to be around when a big old electrolytic shorted out in this kind of supply either.) I'd be concerned about the ripple current rating of the cap too. It might be difficult to find that information on an older cap.

I'd recommend using Schottky diodes. (Lower forward voltage = reduced power dissipation in the diodes)

True... but these can handle 1000V, 25A ;). That's got to be hard to beat :D. Overkill for what is needed though, doubtlessly. It would work really good too if you could just integrate the variac directly into the power supply, like getting a 10 to 1 turns ratio or something variac.

I just looked through the article. I'm unimpressed.

Along with agreeing with your comments in the OC forums, I'm appalled that he never mentioned that the rectified DC output is NOT the same as the rms AC output of the transformer. At 12 V the diode drops will disguise that omission, but at 20+ it'll get interesting. I wonder how many people will kill their TECs.

Yes, I probably should have ripped it more than I did. (Probably would have 'gotten away with it' at that time. I'm probably on the moderator's due for banning list now. Most threads I've written in at oc-forums lately have been deleted.)

Have you been spankin' the kiddies? ;)

Seriously, I wonder why they publish some of their articles. Editing and peer review would be nice addition to the site.

Edit: grammar/spelling

wow...posted that at work yesterday, and when I chk back this morning there is all this good info in there.

Thanks guys

OK...a little itme to digest this.

Truth is, I just don't understand enough of what you guys are saying.

I'm gonna print this out and take it to my EE buddy and see if he can explain it all to me. I hate to take him every little thing, I fear he will get sick of teaching me stuff =P
Then...I'LL BE BACK.

how about THIS, with modifications of course. I assume it is designed for UK mains, but what the hell do I know? and wth is with the car batteries?

I think this should be enough to get my friend an idea of what I wanna do so I can start learning.

Once you get past the messed up link...yup it's a linear power supply.

It's an acceptable basic design.

Just choose a transformer that matches your local main voltage. The transformer will the the biggest, heaviest, most expensive part of the project.

I don't know why it uses a transformer with such a high output voltage. A lot of power is being wasted heating up transistors. I'd use a one that outputs 12V/40A and beef up the first capacitor from 47,000uF to 100,000. Spend money on caps; save on electricity.

Rectifier diodes capable of handling 100A peaks were specified. I'd go with one that can handle 35A average forward current, and 300+ amps of peak current.

The car batteries thing is just for testing/explanation purposes. It would be a bit silly to use two 12V batteries to get 12V.

heh, sry for the screwy link. It was the first time I tried that. Don't post a lot.

Thanks for the info, I'm gonna try to build this thing.
Then I'll prolly add it to the huge pile of useless crap that I've built and never used taking up 1/3 of my garage :)

i've been kinda looking into this, not a lot, but it's something that i'm entertaining.

i found some 12.6v/25a transformers, but i'm guessing from what someone said, this isn't recommended for a 25a psu?

You could squeeze a final ouput of 20A with that transformer. It might get warm, and it might hum from the the high current pulses charging the cap.

Where'd ya find 'em?

I would go for it, if I were you. That would probably be good enough to run a 226W TEC off of. And really, a supply can be even simpler than the one shown above, if you want to use ridiculously huge capacitors. You will also get a very smooth supply if you do that.

all electronics. i used to get their catalog. they have a lot of wierd junk, mostly surplus stuff. it was the biggest transformer they had, but it was like $15 or something. i'm pretty sure they were new.

I can sell you some big power filter caps:
7x 31000mfd 15 VDC, 18 VDC surge
7x 16000mfd 50 VDC, 65-75 VDC surge
2x 61000mfd 10 VDC, 15 VDC surge
2400mfd 200 VDC
35000mfd 40 VDC, 50 VDC surge
39000mfd 7.5 VDC, 9 VDC surge
2100mfd 35VDC, surge 50VDC

Btw, pelts work a hell of alot better at 15 or even 20 volts than they do at 12.

That only applies to TECs that are designed to run at those voltages... if the VMax is 15.2V, running it at 20V is asking for trouble.

I'm assuming those are µF, and not mF, since otherwise, those are some pretty huge capacitors :eek:.

But you will have better results if all your caps are rated for voltages somewhat higher than the maximum output voltage of your rectifires.

Grayson

KnightElite:most ppl commonly use "m" to represent microfarad. I'd like to see a 31000 megafarad("Mf") capacitor...

Grayson:I dont understand what you are trying to say. A 2000 microfarad capacitor is a 2000 microfarad. Now yes it is has a insulation rating. A 1000 volt 2000 mfd will not charge up to 1000 volts with a 12 voltinput if thats what you are thinking. If what you are trying to say is that most rectifiers have a higher voltage output under no load than their input that is true. But it will not be a situation of "better". If the voltage is too high the insulation will be pierced and will destroy the capacitor. Another interesting fact: some capacitors work better around their rated voltage. 2x 25000 microfarad 70 volt capacitors gave me poorer regulation that 2x 24000 40 volt capacitors gave me on my 20 volt psu(drops to about 13 under heavy load).

It is not the voltage rating that determines how well the capacitor filters the ripple. It is the Equivalent Series Resistance (ESR) of the cap. These are independent issues, and a higher voltage cap won't necessarily have a higher ESR.

Hydrogen18: Actually, MF is generally used in the capacitor industry to represent µF, while mF would be milliFarads. Just a note ;). Dunno why they don't just add a µ (ALT+0181) to their printers and do it correctly, but whatever.

I think that what Grayson is saying is to choose capacitors that are rated for slightly more voltage than you will need. I agree, running things at their max tolerance is not advisable for this (only for computer hardware ;)).

lol, we need some standards not involving symbols liek µ then it seems. so how would I symbolize megafarad(Although that is pretty unrealistic, but 1 farad capacitors were unrealistic until recently)

I have a line on some old equip. Caps and such. This stuff comes from a tenant who can't pay rent and is willing to trade to defer some cost. I don't know how to tell if it is gonna be any good but I can pick and choose what I want from a fairly large selection. Probably 2k+ misc components total. Hope he has what I need. Some of the stuff is obviously pretty old and has just been sitting, warehoused, for quite a while. He builds outdoor speaker systems for churches. He has some pretty heavy duty things but plenty of finer equipment as well. Hopefully I can get more specifics as I select what I want. Would make the whole project cheaper.

In addition, this same guy has an old-school o-scope. I mean really old. Looks to be at least 20-25 years old, maybe older. What the hell do I know? Anyway, it is all analogue. Just dials and switches and stuff. No digital lcd displays or any of that cool stuff like I have seen on the new ones. Just a plain old o-scope. He is willing to part with it "cheap", or so he says. We haven't talked price yet. Any pointers on how to tell if it is worth anything? Or what would be a fair price? (assume it works) Perhaps I don't want an old one?

Kind of bogging down on a couple of points. Step-down transformers are so expensive as to make this whole project silly. Variacs are worse :P

Since87, pm about the variac?

thanks for all the info guys.